The marmoset, a small South American primate, is a unique animal in which to study the early embryology of multiple births, and the differentiation of germ cells from early fetal life through adulthood. The marmoset normally produces biovular twins which show complete chorionic fusion prior to somite formation. The embryological processes which lead to blastocyst fusion and early blastocyst form, i.e., prior to fusion, are unknown. It is the aim of this research to describe the pre-fusion blastocyst and determine how and when blastocyst fusion occurs. As a first step in carrying out this study, it will be necessary to determine when, during the marmoset reproductive cycle, ovulation occurs, and to develop a simple and reliable test for early pregnancy. During pregnancy in the marmoset, inter-twin vascular anastomoses develop and allow the exchange of cells which migrate or are motile during embryonic development. This gives rise to individuals which are chimeric for hematopoietic, lymphoid, and, most especially, germinal tissue. Thus, we have available a primate model in which to examine sex-determining factors and in which to evaluate the roles of somatic and germinal elements in gamete differentiation. Data indicates that approximately one-half of all males possess germ cells with an XX sex chromosome constitution. Preliminary evidence indicates that germ cell chimerism also occurs in female marmosets.